This invention relates to a process for treating a liquid containing an Au-CN compound and optionally an Ag-CN compound to decompose the compound(s) and separate gold and optionally silver from the liquid.
When gold and/or silver are contained in industrial effluents, their separation and recovery from the effluents are desired because of an economical reason. Since, however, the gold and/or silver present in industrial effluents are generally in the form of chemically stable cyanides, their separation and recovery are not always accomplished with ease.
With greater attention directed to pollution as a social problem in recent years, more stringent standards have been set for the discharge of industrial effluents containing pollutants. CN-containing waste liquids are also under regulation with increased severity. Accordingly, waste liquids containing Au-CN compounds and/or Ag-CN compounds must be treated for the decomposition of the compounds in addition to the separation and recovery of gold and/or silver therefrom.
For this purpose, liquids containing Au-CN compounds are treated usually by the following two-step process: i.e., (a) zinc is added to such liquid whereby Au in the Au-CN compound is substituted with Zn to precipitate gold, followed by separation; and (b) the zinc cyanide formed through the above substitution is then decomposed with sodium hypochlorite. By such process, gold is usually collected in a recovery rate of about 70 to 80%.
The above conventional process has the following drawbacks:
(1) The recovery of gold and the decomposition of zinc cyanide are performed separately; this is inefficient.
(2) The substitution of Au with Zn needs to be conducted in a vacuum for avoiding the reduction of the efficiency in such substitution due to oxygen; the performance of the entire operation up to the separation of the gold precipitate from the reaction mixture in a vacuum is very cumbersome and requires an expensive apparatus.
(3) The Au concentration and the CN.sup.- concentration in the liquid are limited at most to several thousands of ppm, because the higher the Au concentration is, the lower the substitution efficiency tends to be, and the higher the CN.sup.- concentration is, the higher the concentration of zinc cyanide becomes in the liquid after the substitution, therefore the lower the decomposition efficiency of zinc cyanide by sodium hypochlorite tends to be. Liquids of higher Au or CN.sup.- concentrations need dilution for substitution or decomposition and therefore require a longer period of time for the treatment.